The 3rd Generation Partnership Project (3GPP) defines list information that a user equipment (UE) needs to maintain at a non-access stratum (NAS), such as an equivalent public land mobile network (EPLMN) list, a list of forbidden location areas, a list of forbidden public land mobile networks (PLMNs), and a list of forbidden PLMNs for a GPRS (General Packet Radio Service).
A network side delivers an EPLMN list in an attach accept message or a location update accept message to UE, and the UE stores the EPLMN list. For all radio access technologies (RAT), the UE stores, at any time, only a latest EPLMN list received from the network side. If the network side rejects a registration request of the UE because a location area in which the UE is located is not allowed, the UE adds the current location area in which the UE is located, to a corresponding list of forbidden location areas. If the network side rejects a registration request of the UE because a network is not allowed (PLMN not allowed), the UE adds an identity of the current network to a list of forbidden PLMNs. If the network side rejects a registration request of the UE because GPRS services are not allowed in this network (GPRS services not allowed in this PLMN) or EPS (evolved packet system) services are not allowed in this network (EPS services not allowed in this PLMN), the UE adds an identity of the current network to a list of forbidden networks for a GPRS service (Forbidden PLMNs for GPRS service).
The 3GPP also defines a method for selecting a suitable cell by the UE. The suitable cell is a cell in a PLMN currently selected by the UE, or is a cell in a PLMN with which the UE currently registers, or is a cell in an equivalent PLMN of the UE. In addition, a location area of the suitable cell is not in the list of forbidden location areas. The 3GPP further defines that the NAS does not need to perform a PLMN selection procedure for an inter-EPLMN cell reselection, and that an AS (access stratum) may implement seamless moving between EPLMNs by performing a cell reselection.
In the process of selecting the suitable cell, the NAS of the UE provides the maintained EPLMN list and the maintained list of forbidden location areas to the AS of the UE. In addition, in the process of selecting the suitable cell, the NAS provides information about a currently used RAT to the AS. Different operation modes may be configured for the NAS of the UE: supporting only CS (circuit switched) service, supporting only PS (packet switched) service, or supporting CS service and PS service. For UE supporting CS service and PS service, whether CS service is preferred or PS service is preferred may be further configured.
The UE can camp on a suitable cell in an EPLMN by performing a suitable cell reselection at a lower layer, without initiating a network selection procedure. When this EPLMN is also included in the list of forbidden networks or the list of forbidden networks for the GPRS service, the UE enters a limited service state, that is, the UE can normally camp, but cannot obtain a normal service, and therefore, user experience is affected.